Fix implicit type conversions and warnings on windows builds

The visual studio windows builds spit dozens of lines of warnings
on these.

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>

src/string.c

@@ -67,7 +67,7 @@ struct ssh_string_struct *ssh_string_new(size_t size)
         return NULL;
     }
 
-    str->size = htonl(size);
+    str->size = htonl((uint32_t)size);
     str->data[0] = 0;
 
     return str;